1. Field of the Invention
The present invention relates to a detector circuit used in a wireless communication system, such as a mobile telephone. More particularly, the present invention relates to detection of an operating current in a radio frequency power amplifier for controlling an output power in a wireless communication system.
2. Description of the Background Art
As for mobile telephones, it is necessary to control an output power of a radio frequency power amplifier according to a signal from a baseband circuit, so that the output power from the radio frequency power amplifier is detected. In general, as disclosed in Japanese Laid-Open Patent Publication No. 2001-16116, the output power is monitored in the detector circuit via a coupled device, such as a coupler, converted to a voltage value and fed back to the control circuit and the like.
The detector circuit has been often formed as a semiconductor integrated circuit independent of the radio frequency power amplifier. However, owing to a demand for reduction of the number of parts in recent years, the detector circuit tends to be integrated in the radio frequency power amplifier.
The detector circuit is classified, into a circuit for power detection, a circuit for voltage detection and a circuit for current detection. A conventional detector circuit will be described with reference to the drawing. FIG. 12 is a diagram illustrating a conventional detector circuit of power detection type disclosed in Japanese Translation of PCT International Publication No. 2003-531547.
As shown in FIG. 12, an input matching circuit 2 is connected to a base of an amplifying transistor 1 and an output matching circuit 3 is connected to a collector of the amplifying transistor 1. An input terminal 4 (Vin) is connected to the input matching circuit 2. The output matching circuit 3 is connected to an output terminal 5 (Vout). A bias circuit 6 for supplying a bias is connected to the base of the amplifying transistor 1. A detecting transistor 7 is connected in parallel with the amplifying transistor 1. A root mean square circuit 8 is connected to a collector of the detecting transistor 7. The detecting transistor 7 and the root mean square circuit 8 form the detector circuit 10. The size of the detecting transistor 7 is smaller than that of the amplifying transistor 1.
Next, an operation of the conventional detector circuit 10 will be described.
A radio frequency signal (RF signal) is inputted to the detecting transistor 7 and the amplifying transistor 1 at the same time.
If an output voltage from the detecting transistor 7 is directly used to a detection signal, an accurate average power cannot be obtained since the output voltage temporally fluctuates. For this reason, a voltage proportional to the square of the output current from the detecting transistor 7 is generated and then the generated voltage is averaged, by connecting the root mean square circuit 8 with an output of the detecting transistor 7. Because the transistors used for the amplifying transistor 1 and the detecting transistor 7 have the same structure, the detection voltage (VDET) obtained at the detector output terminal 9 is proportional to the power level of the amplifying transistor 1, and the power level is proportional to the square mean value of the current of the amplifying transistor 1. Accordingly, the power level can be indicated more accurately.
However, the conventional detector circuit 10 detects the inputting power to the amplifying transistor 1. When the load impedance seen by the radio frequency power amplifier changes due to factors such as the distance between the antenna and the user's body, a correct power level cannot be indicated, because the detector circuit 10 cannot follow the load variation.
In addition to this recent digital wireless communication systems are operated under the various modulation signals like Release 99, HSDPA, HSUPA, and various peak-to-average power ratio signals are used. However, when the peak-to-average power ratio of the modulation signal changes, 1 dB gain compression output power (P1 dB) of the transistor changes. Because the radio frequency power amplifier has multiple-stage transistors, the fluctuation of P1 dB causes a difference in the distortion characteristics in the each stage. As a result, output characteristics fluctuate in accordance with the modulation signal. Accordingly, the correct power cannot be obtained by detecting the input power level of the radio frequency power amplifier.
On the other hand, when an output power level of the amplifying transistor 1 is detected, a coupled device, such as a directional coupler, is used. As a result, a circuit scale is increased and it becomes hard for the detector circuit 10 to be incorporated in the radio frequency power amplifier. In addition, such coupled device causes a loss, which has a worse hand in size and performance.